Version 0.18

• #98: Implement JsonPropertyOrderAttribute on records.

• #108: Allow serializing unnamed union fields using their type name, instead of Item1 etc, using JsonUnionEncoding.UnionFieldNamesFromTypes.

• #100: Allow unionTagName to be part of the record type definition when deserializing union of records.

• #109: Add .xml with documentation comments to nupkg. Thanks @exyi!

• #110: Allow customizing the naming policy for union fields with unionFieldNamingPolicy.
  If not provided, use JsonSerializerOptions.PropertyNamingPolicy as before.